The thirty years' truce which was entered into after the conquest of Euboea lasted fourteen years. In the fifteenth, in the forty-eighth year of the priestess-ship of Chrysis at Argos, in the Ephorate of Aenesias at Sparta, in the last month but two of the Archonship of Pythodorus at Athens, and six months after the battle of Potidaea, just at the beginning of spring, a Theban force a little over three hundred strong, under the command of their Boeotarchs, Pythangelus, son of Phyleides, and Diemporus, son of Onetorides, about the first watch of the night, made an armed entry into Plataea, a town of Boeotia in alliance with Athens.

The gates were opened to them by a Plataean called Naucleides, who, with his party, had invited them in, meaning to put to death the citizens of the opposite party, bring over the city to Thebes, and thus obtain power for themselves.

This was arranged through Eurymachus, son of Leontiades, a person of great influence at Thebes. For Plataea had always been at variance with Thebes; and the latter, foreseeing that war was at hand, wished to surprise her old enemy in time of peace, before hostilities had actually broken out. Indeed this was how they got in so easily without being observed, as no guard had been posted.

After the soldiers had grounded arms in the marketplace, those who had invited them in wished them to set to work at once and go to their enemies' houses. This, however, the Thebans refused to do, but determined to make a conciliatory proclamation, and if possible to come to a friendly understanding with the citizens. Their herald accordingly invited any who wished to resume their old place in the confederacy of their countrymen to ground arms with them, for they thought that in this way the city would readily join them.

On becoming aware of the presence of the Thebans within their gates, and of the sudden occupation of the town, the Plataeans concluded in their alarm that more had entered than was really the case, the night preventing their seeing them. They accordingly came to terms, and accepting the proposal, made no movement; especially as the Thebans offered none of them any violence.

But somehow or other, during the negotiations, they discovered the scanty numbers of the Thebans, and decided that they could easily attack and overpower them; the mass of the Plataeans being averse to revolting from Athens.

At all events they resolved to attempt it. Digging through the party walls of the houses, they thus managed to join each other without being seen going through the streets, in which they placed wagons without the beasts in them, to serve as a barricade, and arranged everything else as seemed convenient for the occasion.